An essential step in the commercial production of nodular cast iron is the addition of magnesium to the molten cast iron. The magnesium acts as the nodularizing agent which insures the graphite is precipitated as discrete spheroidal particles in the matrix. The difficulty in adding magnesium to a bath of molten cast iron at 2700.degree. F., is that the magnesium melts at 1200.degree. F., boils at 2200.degree. F., and has a high vapor pressure.
There have been many techniques developed in the past to economically alloy molten cast iron with the volatile, highly reactive magnesium. One reason there are so many prior art processes is that the environment in which the magnesium is added to the molten cast iron directly controls the type of method for introducing additives into the molten cast iron. For example the following series of U.S. patent assigned to the Caterpillar Tractor Company for introducing additives to a casting mold containing molten cast iron would not be applicable to treating molten metal in a ladle: U.S. Pat. Nos. 3,921,700; 3,991,808; 3,991,810; and 4,040,468. The reason that a process for introducing magnesium additives into a casting mold would not work in a ladle containing molten metals is that there are many different variables to consider when comparing the two types of processes, such as: the volume of molten metal, the quantity of magnesium additive, the treatment time, etc.
This invention relates to the commercial production of nodular cast iron by the addition of magnesium to molten cast iron in a ladle. Many ladle methods treatments have been revised for adding magnesium to molten cast iron. The following three described methods are representative of such prior art ladle processes.
In U.S. Pat. No. 2,577,837 to Zifferer, there is disclosed the technique for introducing magnesium wire beneath the surface of a molten cast iron through a pressurized submerged refactory tube. This process has the obvious disadvantage that a pressurized submerged refactory tube must be employed which is both cumbersome to work with and expensive to use.
The second prior art ladle process is disclosed in U.S. Pat. No. 3,768,999 to Ohkubo et al. This patent discloses coating a wire with additive components and an organic binder which thermally decomposes to a gaseous product when added to the molten metal. Obviously it is expensive to construct such a coated wire which prohibitively increases the cost of the treatment process.
A third prior art process is disclosed in the May, 1975 issue of the publication Modern Castings, in an article entitled "The Use of Magnesium Wire Injection for the Production of Nodular Iron" by M. C. Ashton et al. This article discloses the injection of magnesium wire through the bottom wall of a specially constructed ladle. In order to keep the hole through which the wire is fed up into the molten metal it is necessary to maintain a high gas pressure stream through the bottom of the ladle. This utilization of the gas stream has the further disadvantage of producing excessive agitation of the molten iron which contributes to excessive heat losses.